A spell with multiple targets will only fizzle if NONE of the targets are valid at the time of resolution. So if you choose "and," it won't "go back to or," if something makes a target invalid, but it will still hit all valid targets remaining.

Question: Say you were to choose to target two creatures. If one of them then becomes an invalid target (say the opponent gives one of the creatures pro-red at instant speed), does that counter the entire spell or would the other target still be affected? I'm curious if, when you choose the "and" clause, it can no longer go back to "or" or not.